10. Ken Skorseth to Andrew GiesenMessage sent to Andrew Geisen, Washington County, MN on 12-9-2014
Hi Andrew. I didn't blow this off, but just have way too many "irons in the fire" as old-timers say. Here
is some quick input.
I agree that the situation you face is difficult to say the least. Paving would solve the sediment problem,
but do you think that might also bring a couple of new problems — specifically some increase in speed
and immediate, very slippery conditions if snowpack or ice accumulates on the surface? Since it serves a
boat landing, maybe there is little or no traffic after freeze-up? If so, the snowpack or ice problem is
not an issue. Getting back to gravel:
Here is my best advice in constructing a gravel surface on that road.
• Establish good surface shape with crown at or near 4% on the tangents and good shape on the
superelevations (if there are curves in the road) to make sure water drains to the side and not
down the wheel paths of the road. 2% crown normally used on pavements won't get water off
of an aggregate surface, hence the reason for the crown recommendation.
• Next, specify a surface gravel with a closely controlled gradation. I would recommend 1) three-
quarter inch top size control. Larger aggregate is harder to keep embedded in the surface. 2) A
good overall gradation thereafter is needed, but pay special attention to the #200 sieve. I would
set a band of 8 to 15% passing that sieve to make sure you get enough fines to provide a dense,
tightly bound surface. 3) Set a specification on the Atterberg Limits or Plasticity Index of
minimum 4 and maximum 10. This will assure the fine material will have a cohesive
characteristic — in other words, the fines will consist of some natural clay and not be silt or
crusher fines only. Natural clays are remarkable in that they will shed water and actually resist
erosion compared to silt fines. It will also reduce the formation of corrugation or "washboards"
in the surface. I inserted a suggested gradation in a table below.
• I'm not sure what your aggregate sources are, but quarry material with 100% fracture on the
aggregate is definitely the best for this situation.
• The erosion control at the roadside to combat erosion and slow the flow of water in the ditch
would be essentially the same as for pavement on a rural section. Once built, only do blade
maintenance when necessary. If the road is in a reasonably tight, bound state, let it
be. Sometimes operators want to do blade maintenance on a calendar schedule or simply
because they are nearby on another route. Unnecessary blade maintenance can work against
you by loosening stone and fines that will erode away under traffic and heavy rain.
Suggested Surface Gravel Gradation & PI
Sieve Size
Percent Passing
3/4 inch
100
No. 4
55-78
No. 8
40-67
No. 40
18-35
No. 200
8-15
Plasticity Index Range: 4 - 10
Andrew, I also attached a couple of other things. First, an example of an extremely difficult situation on
a road in the Black Hills at Rapid City, SD. The grade on this road is absolutely unmanageable, but the
county is stuck with it. I was on that road and made some observations for a different reason than
making design recommendations, but it may help you. My purpose was to defend what my county
friends did on that road. The road performs remarkably well considering the situation. It evolved from
an old logging road. Today, there are 11 homes on this section and the county is stuck with it. It is not
treated or stabilized in any way.
My final input is on that matter. You mentioned some suggestions on stabilization. This is also
hard. Two things come to mind. I've been involved in many projects using chlorides — either
magnesium or calcium. If you do a few simple things right, it works. The concepts are simple, but
getting it done in the field can be difficult. Again, you have to have good surface gravel in place that
meets the gradation and Atterbergs or PI as shown above. The rest of the process is not easy to define
in an email, but I could give you some further advice at another time if you think that might be an
option. One caution: there is some objection to chlorides being used near water. I haven't had a
problem as long as you get them incorporated and confined in the gravel. Nevertheless, some people
may object. I attached two photos of a M902 stabilized gravel road (Elk Vale). When it remains so
tightly bound a vehicle leaves a skid mark, it's as good as you can hope for.
The other method that has worked well is Portland cement stabilization. We've used that in the ND oil
fields and it works well. It will keep gravel confined and stable under heavy traffic. However, I haven't
seen that on 8 to 10% grades. I'm also a little concerned it may not keep the surface confined as well as
you want. Once constructed, it is hard to do blade maintenance. I attached one picture of a job over in
Cass County, ND. That's not in the oil patch, but they did cement stabilization for to serve heavy ag
traffic on that section. The wheel paths are so tightly bound you can barely drive a nail in
them. However, over time some aggregate loosens. I'm torn on recommending this in your
situation. This one was two years old and remained completely stable under traffic with the exception
of the loose aggregate that accumulated between the wheel paths.
There are over 190 other named products marketed in the US for unpaved road stabilization or dust
control at this time. Most of them are proprietary and it's hard to figure out how to recommend
compatible aggregates since nearly all of the vendors won't reveal their exact chemical formulation. I've
been involved with tests of quite a few of them, but am not comfortable recommending any specific one
for this project.
I'll stop at this point. I hope this at least helps you in thinking through some options. Have a good day.
Photos attached as well as "Observation on Cabot Hill Rd -Pennington Co.pdf':